Infrared detection and imaging device with no moving parts

ABSTRACT

A device images radiation from a scene. A detector is sensitive to the radiation in a first wavelength band. A lens forms an image of the scene on the detector. A filtering arrangement includes two sets of radiation absorbing molecules. A control unit switches the filtering arrangement between two states. In the first state, all of the radiation in the first wavelength band is transmitted to the detector. In the second state, the radiation in a second wavelength band within the first wavelength band is absorbed by the radiation absorbing molecules. The control unit synchronizes the switching of the filtering arrangement with the detector. Each pixel of the image formed on the detector includes two signals. The first signal includes information from the scene radiation in the first wavelength hand. The second signal excludes information from the scene radiation absorbed by the filtering arrangement in the second wavelength band.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional PatentApplication No. 62/077,328, filed Nov. 10, 2014, whose disclosure isincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to the detection and imaging of infraredradiation.

BACKGROUND OF THE INVENTION

The last twenty five years have seen a very large amount of work done inthe field of multispectral and spectral imaging all around the world, inindustry and academia. The motivation is to collect spatially andspectrally resolved radiance information of a predefined region ofspace. The number and fields of application of such information areinnumerable; as a result, the different optical configurations used inthe design of instrumentation built for this purpose have also beeninnumerable, in order to provide the needed information through a methodand format most suitable for the relevant application. Instrumentationhas been built for laboratory and field use, industrial and militaryuse, on land, on the sea, from the air and in space. Such types ofinstrumentation are based in the visible spectral range (400-750nanometers), and in the various regions of infrared (the Near Infrared(NIR) range of 750-2500 nanometers, the Mid Wave Infrared (MWIR) rangeof 2500-5000 nanometers, and the Long Wave Infrared (LWIR) of5000-14000). Instrumentation of spectral imaging has been built foranalysis of microscopic samples in hospital environments, as well as ofdistant cosmic objects through large astronomical telescopes. The sizeof the analyzed region of space and spatial resolution also vary widely,as well as the spectral resolution, depending on the type of detectorused (i.e. the size, speed, sensitivity and number of resolutionelements (pixels) that the detector provides). All work done in thisfield allows the acquisition of knowledge of the spatial distribution(the imaging side) of the material constituents (the spectroscopic side)in a predefined region of space. Just one of many examples of earlycommercial spectral imagers is the microscope mounted SpectraCube 1000of the early nineties (“Novel spectral imaging system combiningspectroscopy with imaging applications for biology”, Proc. SPIE Vol.2329, 180 (1995), and U.S. Pat. No. 5,539,517), sensitive in the visiblerange, invented at CI Systems in 1991 and transferred to Numetrix andlater to Applied Spectral Imaging Ltd. for use in medical applications.

The early drive in the 1990's was to make the transition from systemsbased on imaging at a limited number of wavelengths using bandpassfilters (below ten) used on satellites (like Landsat and similarinstruments of the 1970's and 1980's), to hyperspectral imaging,providing hundreds of spectral resolution elements at each image pixel,using gratings and interferometers and sophisticated optics. The lattergroup, especially through advances in modern cooled or chilled CCD's andcryogenically cooled infrared detector arrays, represents a high endtype of instrumentation. Such high end instrumentation provides largequantities of information, but is usually more suitable for researchprojects and not for practical day-to-day civilian and industrialapplications, due to exceedingly high costs which may reach hundreds ofthousands of dollars each.

A more recent trend is to exploit spectral imaging technology forsafety, security and industrial applications, and in particular in theapplication of hazardous gas cloud detection and imaging. Spectralimaging technology as applied to such applications can be used, forexample, in automatic detection of leaks in industrial installationswithout the need for manpower intensive maintenance investigations, andto identify gases liberated to the air in traffic accidents involvingtrucks during transport. The low price and maintenance-free operationrequired for this type of instrumentation is a strong motivation to uselow price detectors with no moving parts. Both are significantchallenges: i) the former, because most hazardous gases in safetyapplications are transparent in the visible range and require moreexpensive infrared detectors, in order to be detected; ii) the latter,because the need for simultaneous spatial and spectroscopic informationneeded for detection and identification is more easily achieved with aspectral scanning method of some kind, usually requiring the movement ofsome optical component, such as a scanning mirror of an interferometer,a set of band pass filters mounted on a rotating wheel, or other.

Recent technological advances have allowed the development of more costeffective infrared detectors and cameras not requiring cooling at all,or at most requiring thermoelectric cooling (for example microbolometersMIR detectors and MWIR PbSe arrays). As a result, the motivation to findoptical configurations yielding just the right amount of information fora specific application, while maintaining low cost, has also become verystrong. Compromises in this respect have been described in theliterature of many different types. Dereniak et al., (“Snapshotdual-band visible hyperspectral imaging spectrometer”, OpticalEngineering Vol. 46(1), 2007) and Kudenov et al., (“Review of snapshotspectral imaging technologies”, Optical Engineering 52(9), 090901(September 2013)), discuss the use of Optical Computed Tomography (OCT)techniques to remove the use of moving parts in spectral imaging at theexpense of spatial and spectral resolutions. Other snapshot methodsdescribed in Kudenov et al., have been developed. Such snapshot methodsstill provide a spectral image with intermediate resolution in bothwavelength and space parameters, but the optical fabrication of therequired exotic components is cumbersome and expensive (such asreformatting coherent fiber bundles, lenslet arrays, and multiple mirrorfacets).

SUMMARY OF THE INVENTION

The present invention is a reliable and low cost device for detecting,imaging and quantifying an airborne gas in a specific range ofconcentration and cloud size. The device uses no moving parts and has anoptical system based on a bistatic electronically controlled notchabsorber, absorbing in the same wavelength range as the gas to bedetected. The device alternately images a field of view through abistatic absorber in the notch and out-of notch wavelength ranges,respectively.

According to an embodiment of the teachings of the present inventionthere is provided, a device for imaging radiation from a scene, theradiation including at least a first and second wavelength band, thesecond wavelength band included in the first wavelength band, the devicecomprising: (a) a detector of the radiation from the scene sensitive toradiation in the first wavelength band; (b) an image forming opticalcomponent for forming an image of the scene on the detector; (c) afiltering arrangement including first and second independentlycontrollable pluralities of radiation absorbing molecules in the secondwavelength band, the filtering arrangement configured to beelectronically switched between: (i) a first state, in which all of theradiation in the first wavelength band is transmitted to the detector,and (ii) a second state, in which the radiation in the second wavelengthband is at least partially absorbed by each of the first and secondpluralities of radiation absorbing molecules, and (d) a control unitelectrically coupled to the filtering arrangement and the detector forsynchronizing the switching of the filtering arrangement with the imageforming on the detector, such that, each pixel of the formed imageincludes: (i) a first signal including information associated with thescene radiation in the first wavelength band, and (ii) a second signalincluding information associated with the scene radiation in the firstwavelength band and excluding information associated with the sceneradiation absorbed by the filtering arrangement in the second wavelengthband.

Optionally, when the filtering arrangement is in the first state, thefirst plurality of radiation absorbing molecules are arranged in a firstorientation substantially parallel to a direction of propagation of theradiation from the scene to the detector.

Optionally, when the filtering arrangement is in the second state, thefirst plurality of radiation absorbing molecules are arranged in asecond orientation substantially perpendicular to a direction ofpropagation of the radiation from the scene to the detector.

Optionally, when the filtering arrangement is in the first state, thesecond plurality of radiation absorbing molecules is arrangedsubstantially parallel to the first plurality of radiation absorbingmolecules.

Optionally, when the filtering arrangement is in the second state, thesecond plurality of radiation absorbing molecules is arrangedsubstantially perpendicular to the first plurality of radiationabsorbing molecules.

Optionally, the first and second pluralities of radiation absorbingmolecules are arranged in series such that the second plurality ofradiation absorbing molecules is interposed between the first pluralityof radiation absorbing molecules and the detector.

Optionally, the first plurality of radiation absorbing molecules arepositioned within a first cell, and the second plurality of radiationabsorbing molecules are positioned within a second cell.

Optionally, the radiation from the scene is unpolarized.

Optionally, when the filtering arrangement is in the second state, theradiation in the first wavelength band is polarized by the firstplurality of radiation absorbing molecules.

Optionally, the first plurality of radiation absorbing molecules isconfigured to polarize the radiation incident from the scene.

Optionally, the second plurality of radiation absorbing molecules isconfigured to polarize the radiation incident from the first pluralityof radiation absorbing molecules.

Optionally, the image forming optical component has an optical f-numberless than approximately 1.5.

Optionally, the filtering arrangement is interposed between the sceneand the image forming optical component.

Optionally, the filtering arrangement is interposed between the detectorand the image forming optical component.

Optionally, each of the first and second pluralities of radiationabsorbing molecules includes liquid crystal molecules.

Optionally, the radiation in the first wavelength band includesradiation in the range of 3.2-3.5 micrometers in wavelength.

Optionally, the first and second signals provide quantitativeinformation about the scene, the quantitative information produced by ananalytical technique.

Optionally, the scene is a gas cloud.

Optionally, the gas cloud is a hydrocarbon gas cloud.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1 is a schematic side view illustrating a prior art device fordetecting and imaging radiation from a gas cloud;

FIG. 2 is a schematic side view illustrating a device for detecting andimaging radiation from a scene according to an embodiment of theinvention;

FIG. 3A is plot of the transmittance of a filter according to anembodiment of the invention;

FIG. 3B is a plot of the transmittance of a gas cloud imaged by thedevice of FIG. 2;

FIG. 4 is a plot of detected radiation for different filtertransmittance values as a function of gas cloud absorption;

FIG. 5 is a plot of the absorption characteristics for a group ofhydrocarbon gases;

FIG. 6 is a plot of the transmission characteristics of a liquidcrystal;

FIG. 7A is an illustration of radiation absorbing molecules in apolarizing state;

FIG. 7B is an illustration of radiation absorbing molecules in anunpolarizing state;

FIG. 8A is an illustration of two cells of radiation absorbing moleculesin a second state in which incident light is at least partially absorbedin the absorption wavelength range of the radiation absorbing molecules;

FIG. 8B is an illustration of two cells of radiation absorbing moleculesin a first state in which incident light is completely transmitted inthe detector wavelength range of sensitivity, including the absorptionwavelength range of the radiation absorbing molecules.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The principles and operation of the device according to the presentinvention may be better understood with reference to the drawings andthe accompanying description.

Before explaining at least one embodiment of the invention in detail, itis to be understood that the invention is not necessarily limited in itsapplication to the details of construction and the arrangement of thecomponents and/or methods set forth in the following description and/orillustrated in the drawings and/or the examples. The invention iscapable of other embodiments or of being practiced or carried out invarious ways. Initially, throughout this document, references are madeto directions such as, for example, right, left, and the like. Thesedirectional references are exemplary only to illustrate the inventionand embodiments thereof.

The present disclosure is of a device for detecting and imaging aspecific gas in the air in a specific range of concentration and cloudsize that may be among the least expensive to build and most reliable,by avoiding the use of moving parts, and for measuring the pathconcentration of the gas in each pixel of the image. This is madepossible, as will be explained in the following sections, by acompromise on the number of wavelength ranges combinations used, comingdown to only two, by the use of an infrared sensitive camera (preferablyuncooled or thermoelectrically cooled such as a PbSe camera sensitive tothe 1 to 5.5 micron range, or microbolometer in the to 14 micron range),and by an optical system based on a bistatic electronically controllednotch absorber, absorbing in the same wavelength range as the gas to bedetected. Other cooled cameras may be used at higher expense. The devicealternately images a field of view through a bistatic absorber in thenotch and out-of notch wavelength ranges respectively. A mathematicalrelation between the two signals (in and out-of-notch) at each imagepixel is then used to calculate the path concentration of the gas makingout the hazardous cloud present in the corresponding region of space, aswill be explained in Section 1 below.

1. Gas Detection And Imaging Using Two-Wavelength Passive InfraredRadiometry:

It has been well known for many years that it is possible to detect thepresence of a gas in the air and measure the corresponding pathconcentration distribution by measuring the infrared self-emission ofthe background of the gas cloud in two different wavelengths, one whichis absorbed by the gas and one which is not, provided that thebackground and gas are not at the same temperature. The radiancedifference R reaching the measuring instrument between the twowavelengths w₀ (not absorbed) and w_(G) (absorbed by the gas), can beexpressed in terms of the background radiance B, the gas temperatureT_(G) (usually equal to the air temperature, and we assume that it isknown by measurement) and the gas transmittance t_(G) at the absorbedwavelength. The gas transmittance t_(G) is in turn dependent on themolecular absorption coefficient of the gas in question multiplied bythe cloud thickness and gas molecular concentration at the pixel inquestion (referred to as the path concentration). Therefore, if the gasin question is known, the gas path concentration can be estimated. Thefollowing will clarify the quantitative method of the present disclosureby showing that t_(G) can be measured with the present device, which inturn allows for the gas path concentration to be estimated:R=B−B*t _(G)−(1−t _(G))*Pl(T _(G) ,w _(G))=(1−t _(G))*{B−Pl(T _(G) ,w_(G))}  (1)where Pl(T_(G),W_(G)) is the Planck function at temperature T_(G) andwavelength w_(G). Two simplifications are used in equation (1) which arenot important for the sake of this explanation because the associatedphenomena can both be calibrated out in the more general case: i)atmospheric transmittance is assumed to be 1, and ii) backgroundradiance in and out of the gas absorption band are equal.

It is obvious from equation (1) that in the case that B is equal toPl(T_(G),W_(G)), the radiance difference R is equal to zero,irrespective of the value of t_(G), and in this case no information canbe inferred on the quantity t_(G). However, if B is different thanPl(T_(G),W_(G)), then equation (1) can be solved for t_(G) as follows:

$\begin{matrix}{t_{G} = {1 - \frac{R}{B - {P\; 1\left( {T_{G},w_{G}} \right)}}}} & (2)\end{matrix}$

All parameters on the right hand side of equation (2) are known: B isknown because it is measured in the non-absorbing wavelength w₀, Pl isknown because T_(G) is measured and w_(G) is known, and R is measured.Therefore t_(G) is known from equation (2). If the molecular absorptioncoefficient, A_(G), of the specific gas being monitored is known fromthe literature at w_(G), then t_(G) gives a measure of the product ofaverage gas volume concentration in the cloud, multiplied by thethickness of the cloud itself, or the so called concentration timeslength (or path concentration) value of the cloud. In fact, by theLambert-Beer law as follows:t _(G) =e ^(−nA) ^(G) ^(l)  (3)where l is the path length or thickness of the cloud and n is theaverage volume concentration of the gas being measured in the cloud,both corresponding to a specific pixel being examined. From equation(3), nl can be expressed as:

$\begin{matrix}{{nl} = {\frac{1}{A_{G}}{\ln\left( \frac{1}{t_{G}} \right)}}} & (4)\end{matrix}$If l is known then the average concentration n can be estimated by:

$\begin{matrix}{n = {\frac{1}{{lA}_{G}}{\ln\left( \frac{1}{t_{G}} \right)}}} & (5)\end{matrix}$

Note that the estimated average concentration n expressed in equation(5) assumes absorption from single molecules, negligible attenuationeffects from scattering, reflections, and multiple absorption in the gascloud. In general, l is not known and A_(G) is known, so this methodreadily provides nl according to equation (4), once t_(G) is measuredaccording to equation (2).

The purpose of the present invention is to give a solution to theproblem of detecting and imaging the concentration times path lengthdistribution of an infrared wavelength absorbing gas cloud (or othersimilar material with an absorbing wavelength) with the minimum use ofmoving parts and at the same time retaining the best sensitivitypossible.

FIG. 1 depicts an example of a prior art solution. In such a solution,an objective lens 2 is positioned in front of a detector array 1 and atwo-position filter holder or wheel 3 containing two filters (4 a,4 b),either in front of the objective lens 2 or between the objective lens 2and the detector array 1. In this way, by recording successive images ofthe field of view (FOV) of the instrument at the two wavelength bandsthe needed information is gathered to detect the gas, if present, and inthis case, calculate the concentration times path value of the gas inquestion for every pixel of the scene, as explained above. From thisinformation the image of the cloud is built pixel by pixel and eitherdisplayed in an enhanced color, or otherwise used, e.g. to issue analarm if this value becomes higher than a minimum safety value in somepredefined spatial criteria. In the example shown in FIG. 1, the filterholder 3 slides in front of the objective lens 2 in a linear motion.Other configurations using rotating wheels, grating, etc. to filter thedifferent wavelengths can be used, but they all use moving parts. Thefilters are before the objective lens. Only the principal rays of thecentral, top and bottom pixels of the FOV are shown. The filters can bealternately placed between the lens and the detector or between lensesin a multiple lens system design.

It should be noted that the configuration of FIG. 1 can be preferablydesigned with a large numerical aperture of the objective lens 2 toexploit the best possible detector sensitivity (or low f-number, whichis in general kept as close to 1 as possible, especially when usingcooled infrared detector arrays). A different configuration, using adichroic beamsplitter to split the incoming beam into two beams to befiltered separately in the two wavelengths and two separate detectorscan be used, but would be more expensive because of the additionaldetector cost. A further similar configuration using, besides thedichroic filter, an additional beam combiner and chopper may be used tolimit the design to the single array detector, but in this case thechopper, needed to switch between the two wavelengths in synchronizationwith the detector frame capture rate, is a low reliability moving part.These last two configurations require more complicated optics to avoiddecreasing the numerical aperture of the focusing optics at the detectorand degrade the system sensitivity.

2. Bistatic Liquid Crystal Solution:

Refer now to FIG. 2, a device 10 of an embodiment of the presentinvention. The device 10 enables imaging the same region of space orfield of view (FOV) alternately through two electronically controlledfiltering states, using a single detector array camera 12 and opticalsystem without mechanical movement of any optical component (the shutteris still needed to carry out NonUniformity Corrections, includingenvironmental drifts, fixed pattern and readout noise), while preservingthe high optical throughput of the conventional configurationspreviously described in the Section 1. This is accomplished by acombination of an imaging optics 14 (referred to interchangeably asfocusing optics, image forming optics, and objective lens) with f-numberclose to 1 (high numerical aperture) and a bistatic liquid crystal (LC)based notch filter 16 (referred to interchangeably as LC filter, LCnotch, LC notch filter, bistatic LC filter, bistatic LC notch filter)having a spectral behavior optimized to the gas absorption spectrum inquestion. The LC filter configurations pair are, as explained in theexample below, i) a high filter transmittance spectral region, and ii) anotch filter absorbing in a region around a wavelength absorbed by thegas, this wavelength being within the high transmittance wavelengthregion. These two configurations are alternately switched electronicallyby a controller 18 in synchronization with the camera frames recording.In this way the device 10 records two signals for each pixel, onethrough the high transmittance and one through the notch configuration.In Section 3 we show how to calculate t_(G) of equation (1) from thesetwo pixel signals.

The controller 18 can be implemented as any number of computerprocessors including, but not limited to, a microprocessor, an ASIC, aDSP, a state machine, and a microcontroller. Such processors include, ormay be in communication with computer readable media, which storesprogram code or instruction sets that, when executed by the processor,cause the processor to perform actions. Types of computer readable mediainclude, but are not limited to, electronic, optical, magnetic, or otherstorage or transmission devices capable of providing a processor withcomputer readable instructions.

Once t_(G) is known in every pixel, equation (4) yields the informationof concentration times path length of the gas cloud at every point inthe image. From this information one can build the image in false coloror in different intensity levels of the color, according to the nl valueof the gas being monitored at every pixel. Capturing alternatinginformation in these two configurations on the same detector 12according to the present invention also allows the monitoring of fasterphenomena than in a conventional configuration, because the electronicswitching between the “off” (high transmittance) and “on” (notchtransmittance) configurations of the filter can be done at highfrequencies of several kHz. This is due to the fast LC moleculesresponse to the suitably applied high frequency alternating voltage.With the proper digital analysis, the final gas cloud image can be shownsuperimposed on a conventional visible image of the same field of view,obtained by a usual visible CCD camera. This helps the operator locatethe cloud with respect to other objects in the field.

With continued reference to FIG. 2, the edge and central rays of the FOVare depicted passing through the two bistatic filter 16 and focusingoptics 14, and forming the scene image on the detector plane. The viewin FIG. 2 is a cross section from the side of the device 10. The movingfilter holder or wheel 3 of FIG. 1 is replaced by the electronicallycontrolled bistatic LC filter system (the bistatic LC filter 16 and thecontroller 18), including a high transmittance configuration in awavelength range, alternating with a notch absorbing filterconfiguration in a sub-range corresponding to the gas absorptionwavelength.

3. Gas Path Concentration Measurement:

As explained in Section 1 with reference to equation (4), if theabsorption coefficient of the gas molecules being monitored is knownfrom the literature and the cloud transmittance is measured, then thecloud path concentration can be at least approximately calculated. Inthis section it is shown how the signals through a bistatic notch filtercan be used to calculate the transmittance through the gas cloud.

In FIG. 3A the transmittances of the bistatic LC filter 16 in the offand on configurations are shown. The dotted line is the “off”configuration (i.e. no notch region). In the “off” configuration, thereis 100% transmittance in the range between λ₁ and λ₂ and 0%transmittance outside of that range). In the “on” configuration, thetransmittance is a notch centered at and width w. The transmittance inthe notch region is z.

In FIG. 3B, the gas cloud transmittance in a pixel of path concentrationnl is shown. The absorption wavelength range is centered in the samewavelength λ₀ as the LC notch and has the same width w. Thetransmittance in the absorption range is x and 100% otherwise.

The detector array 12 is assumed to be sensitive only in the wavelengthregion between λ₁ and λ₂. Now assume that the spectral radiance of thecloud background is B and is a constant function of wavelength. Then, inview of FIGS. 3A and 3B, the average radiance reaching the detectorarray 12 in the wavelength range between λ₁ and λ₂ can be expressed forfour cases. Defining D=λ₂−λ₁ and y=w/D, where w is defined above, thefour cases are defined as follows:

First case: gas present (G), notch on (N):

$\begin{matrix}{\frac{S_{G,N}}{D} = {{{B\left( {1 - \frac{w}{D}} \right)} + \frac{Bwxz}{D}} = {{B\left( {1 - y} \right)} + {Byxz}}}} & (6)\end{matrix}$Second case; gas present (G), notch off (O):

$\begin{matrix}{\frac{S_{G,O}}{D} = {{{B\left( {1 - \frac{w}{D}} \right)} + \frac{Bwx}{D}} = {{B\left( {1 - y} \right)} + {Byx}}}} & (7)\end{matrix}$Third case: gas absent (A), notch on (N):

$\begin{matrix}{\frac{S_{A,N}}{D} = {{{B\left( {1 - \frac{w}{D}} \right)} + \frac{Bwz}{D}} = {{B\left( {1 - y} \right)} + {Byz}}}} & (8)\end{matrix}$Fourth case: gas absent (A), notch off (O):

$\begin{matrix}{\frac{S_{A,O}}{D} = {\frac{ED}{D} = B}} & (9)\end{matrix}$

Equations (8) and (9) are special cases of equations (6) and (7)respectively for x=1. This should be apparent, as x=1 implies that thegas cloud is absent. In this case the right hand side of equation (6) isthe same as in equation (8) and the right hand side of equation (7)becomes equation (9). Similarly, for a very, high concentration gascloud, x=0, and equations (6) and (7) give the same result. This isbecause the total signal is B multiplied by the product of the twotransmittance functions in FIGS. 3A and 3B. If the gas cloud absorbs allradiation within the notch range the signal is independent on whetherthe LC notch is in the on or off configuratio.

A ratio, F, of the difference between “on” and “off” and the “off”signal (or normalized signal contrast) can be calculated from equations(6) and (7). The ratio IF can be expressed as follows:

$\begin{matrix}{F = {\frac{{S_{G,O}/D} - {S_{G,N}/D}}{S_{G,O}/D} = \frac{{yx}\left( {1 - z} \right)}{1 - {y\left( {1 - x} \right)}}}} & (10)\end{matrix}$

Equation (10) is the basic relation between the pixels' signals in theLC on and off positions and the quantity x, which is equal to t_(G) ofequation (3) above, in turn the parameter related to the pathconcentration of the gas cloud in question according to equation (4).

The ratio F of equation (10) can be plotted as function of the total gascloud absorption (ln(1/x)=nlA_(G) of equation (4)) for different valuesof y and z. Example plots are shown in FIG. 4.

The plotted values of FIG. 4 are calculated for y=0.8. (y can change bydefinition between 0 and 1, but these two values are limiting valuesthat have no physical meaning for our purpose). In fact, y=0 is notphysical and for y=1, F does not depend on x. The exact values of y andz should be selected according to the design of the device 10. Inpractice, the values of y and z are typically measured separately in thelaboratory or known from the literature. Once y and z are known, thefunction F of equation 10 is plotted as a function of x for these valuesof y and z. x=t_(G) is then determined by determining the value ofln(1/x) corresponding to the measured value of F for the particularpixel in question on the plotted graph. This can be done, since theplotted functions of F, as seen in FIG. 4, are monotonic, and thereforehave one-to-one relationships. Alternatively, equation (10) can beinverted to express x as function of F, y and z. Again, once F ismeasured with the device 10 and y and z are known independently, x=t_(G)(and therefore is also known.

It is noted that the above example refers to the case in which thebackground of the gas cloud is at higher temperature than the clouditself, so that the gas appears as absorbing. However, the samearguments can be shown to hold in the opposite case of background beingcooler than the gas cloud, when the gas appears as an emitter.

4. Liquid Crystal Implementation

In this section, implementation of the bistatic notch LC filter 16, inorder to provide the type of signals as described above without movingparts, is discussed. This section also addresses the integration of thebistatic notch LC filter 16 in the device 10, in order to detect andimage a gas cloud. The implementation and integration discussion isapplied, as a non-limiting example, to a group of hydrocarbons that allhave an absorption around the same wavelength range, between 3.2 and 3.5microns, and to a particular detector type that is sensitive in thatrange. As should be apparent to one of ordinary skill in the art, manyother combinations of materials to be detected, LC filters materials,and different detectors may be used according to the teachings of thepresent embodiments described herein.

The absorption characteristics for the non-limiting example group ofhydrocarbons are shown in FIG. 5. In the particular wavelength range ofhydrocarbons pertinent to the present non-limiting example, there existliquid crystals, such as, for example, E-7 and BDH-E7. Such liquidcrystals present absorption in this wavelength range. The transmissioncharacteristics of such example liquid crystals are shown in FIG. 6which is an excerpt from the Handbook of Optics, McGraw-Hill Inc.,Sponsored by the Optical Society of America, Volume II, page 14.6, 1995.

As described, for example, by C. L. Mulder et al., in the publicationDye alignment in luminescent solar concentrators: I. Vertical alignmentfor improved waveguide coupling, Optics Express Vol. 18, No. S1, p. A79,2010, there are at least two ways to obtain the same desired effect. Oneis that the LC itself preferentially absorbs infrared radiationpolarized along the long molecular direction and does not appreciablyabsorb radiation polarized perpendicular to that direction, includingwhen the molecule is aligned parallel to the incoming infrared lightpropagation direction. The second way is to have a long absorbingmolecule embedded in the LC matrix, such that its polarizationproperties are controlled in a similar way by the LC moleculesalignment. In both cases a cell containing the LC itself or themolecular mixture mentioned here, can be electronically induced toabsorb radiation anisotropically, as shown in FIG. 7A.

In FIG. 7A, unpolarized light 70 is incident from the left on an LC 72or absorbing long molecule oriented perpendicularly to the lightpropagation direction. The transmitted light at wavelengths within thegas and LC absorption wavelength region is polarized perpendicularly tothe long axis of the molecule. Light outside this band is not absorbedand is unpolarized. As a result, in this example, molecules orientedperpendicularly to the light propagation direction have the followingeffect: they absorb the portion of light around the 3.5 microns rangewhich is polarized parallel to their long molecular axis, whiletransmitting the same wavelength range polarized in a directionperpendicular to their long molecular axis (depicted as a transmittedpolarized light beam 74 around the gas absorption wavelength); inaddition, light outside the 3.5 micron wavelength range is completelytransmitted.

Alternately, when these molecules are placed in a cell and theappropriate voltage is applied to it so that their long axis is alignedwith the propagation direction of the incoming light, they do not absorbany light and in this situation all radiation is transmitted through thecell, as shown in FIG. 7B.

In FIG. 7B, the unpolarized light 70 is incident on an LC long molecule76 whose long axis is oriented in the direction of the light propagationdirection. As a result, all light is transmitted and is unpolarized(depicted as a transmitted unpolarized light beam 78). The configurationdepicted in FIG. 7B is the “off” filter configuration of the LC filter16.

Now consider, as in FIGS. 8A and 8B, a system composed of two equal LCcells in series (a first LC cell 80 and a second LC cell 82), where themolecules 81 in the first LC cell 80 and the molecules 83 in the secondLC cell 82 are oriented with their long axis in two perpendiculardirections with respect to each other and to the light propagationdirection. An unpolarized light beam 84 incident from the left iscompletely (or partially) absorbed in the absorption wavelength range ofthe molecule and gas by the pair of cells (this is the “on” filter stateof the LC filter 16 described in Section 3 above). This is due to thefact that the second LC cell 82 absorbs the residual polarized light 85transmitted by the first LC cell 80 (functioning as cross polarizerswith respect to this radiation). All light outside that range istransmitted. When the molecules of both cells are switchedelectronically (via the control unit 18) to be oriented parallel to thelight propagation direction as in FIG. 8B, no light is absorbed (this isthe “off” state of the LC filter 16 described in Section 3 above).Specifically, in FIG. 8B, the unpolarized light beam 84 is transmittedunpolarized (and shown as 85) after passing through the first LC cell80, and likewise by LC cell 82. As a result, all light incident on thefirst LC cell 80 passes through the second LC cell 82 within the wholewide band region between λ₁ and λ₂ of FIG. 3A.

As should be apparent from the discussion above, the device 10 (as shownin FIG. 2) in which the filtering element 16 is a bistatic filter asdescribed above in Section 4, provides images of the scene in which eachpixel's radiation is known according to the four equations (6), (7),(8), and (9). Accordingly, a procedure as described in Section 3 can befollowed to detect a gas cloud having the above described absorptionproperties, if present in the air, and map the path concentration of thegas cloud pixel by pixel.

It will be appreciated that the above descriptions are intended only toserve as examples, and that many other embodiments are possible withinthe scope of the present invention as defined in the appended claims.

What is claimed is:
 1. A device for imaging a scene, the devicecomprising: (a) a detector; (b) an optical arrangement defining anoptical path from the scene to said detector; (c) a switchable filtercomprising at least one cell of liquid crystal material, said switchablefilter being deployed in said optical path; and (d) a controller forselectively switching said switchable filter between a first state, inwhich molecules of liquid crystal within said liquid crystal materialare aligned in a first orientation, and a second state in which saidmolecules of liquid crystal are not aligned with said first orientation,said first and second states differing in their spectral absorptionproperties, wherein said controller is configured to switch saidswitchable filter synchronously with sampling of pairs of images of thescene formed on said detector, said pairs of images comprising a firstimage formed with said switchable filter in said first state and asecond image formed with said switchable filter in said second state,and wherein the device is configured to co-process a first pixel signalassociated with said first image and a second pixel signal associatedwith said second image to derive a spectrally selective difference pixelsignal.
 2. The device of claim 1, wherein said first orientation issubstantially parallel with said optical path.
 3. The device of claim 1,wherein, in said second state, a majority of said molecules of liquidcrystal are aligned substantially perpendicular to said firstorientation.
 4. The device of claim 1, wherein said switchable filtercomprises two cells of liquid crystal material, and wherein, in saidsecond state, a majority of said molecules of liquid crystal in a firstof said cells are aligned in a second orientation substantiallyperpendicular to said first orientation, and wherein, in said secondstate, a majority of said molecules of liquid crystal in a second ofsaid cells are aligned in a third orientation substantiallyperpendicular to both said first orientation and said secondorientation.
 5. A method comprising: (a) deploying in an optical path ofan imaging device a switchable filter comprising at least one cellcontaining liquid crystal material; (b) forming a first image on adetector of the imaging device while the switchable filter assumes afirst state in which molecules of liquid crystal within the cell arealigned in a first orientation; and (c) forming a second image on thedetector of the imaging device while the switchable filter assumes asecond state in which the molecules of liquid crystal are not alignedwith the first orientation, said first and second states differing intheir spectral absorption properties; and (d) co-processing a firstpixel signal associated with said first image and a second pixel signalassociated with said second image to derive a spectrally selectivedifference pixel signal.